DIY Stompboxes => Building your own stompbox => Topic started by: Eddward on September 21, 2019, 12:20:33 PM

Title: Middle C/261hz hum?
Post by: Eddward on September 21, 2019, 12:20:33 PM
Hi all,

I've noticed the following in 2 of my boxes...

EHX Small Clone (stock), big box - when there is no signal from the guitar (i.e. tuning bypass, turning the volume fully down, or using a killswitch), there is a continuous tone. I've found it's near enough middle C, 261hz. I haven't tested this with a battery in place of an isolated power supply.

K&K Trinity System Preamp - I've modded it to include an external DC input (pedal board style), so it can run on battery as usual or be plugged via a standard pedal adaptor. No problem on the battery, but when plugged into the isolated power supply, the middle C sine wave returns! This time, the problem is only on the mic signal, and it is only audible when the mic is turned on (it is therefore present during performance).

This doesn't occur in any of my other units using the same isolated supply! Modded or stock, self build or professionally built, the problem only occurs in the above cases.

Question, is there a simple circuit I can place somewhere in the pedals to remove this unwanted noise?

It is a particular nuisance in the K&K Preamp. I dislike using batteries as I have no faith in myself to check and change them regularly enough! Much prefer a solid, stable, mains connection.

Thanks in advance,
Title: Re: Middle C/261hz hum?
Post by: Walto on September 21, 2019, 02:45:15 PM
Taking a stab at this. It sounds like it might be a frequency from the power supply, either actually 261Hz, or a higher frequency modulated down to the audio range when it interacts with the circuits in those devices.

To verify, easiest is probably to just try a different make/model of power supply.

If you don't want to replace it, you could try additional filtering inline between the power supply and the pedals.

Title: Re: Middle C/261hz hum?
Post by: GibsonGM on September 21, 2019, 03:22:08 PM
Clip a battery in there (watch your polarity!) to verify it's the power supply.  If it's not meant for audio equipment, it probably is.  You can try a 10R resistor in series where power comes in, with a 470uF or bigger cap to ground, might help filter it.
Title: Re: Middle C/261hz hum?
Post by: Eddward on September 22, 2019, 12:18:03 PM
Thanks both,

It is a relatively cheap PSU, but it is meant for guitar effects (it's a Mosky Powerstation from Amazon). I'm not too keen on replacing it as it works so well with everything else!

GibsonGM - both units have a stock battery clip inside, as mentioned the preamp sees no issue using battery only. When adding in the 10R resistor and 470uF cap, would I be looking to solder them directly to the power socket, and then the circuit +/- to those respectively?


+ -> 10R Resistor -> circuit positive
- -> 470uF cap -> circuit negative

In series therefore, right on the input, rather than in paralell?

Thanks again,
Title: Re: Middle C/261hz hum?
Post by: GibsonGM on September 22, 2019, 12:51:01 PM
Like this, Edd!    The absolute values are not that critical, but of course a bigger cap will give a bigger 'reservoir'.   Some use 1000u.  A slightly bigger R would lower the cutoff frequency, offering more rejection.